Coronary artery disease (CAD) is a narrowing of the arteries that supply the heart with blood carrying oxygen and nutrients. CAD may cause shortness of breath, angina, or even heart attack. The narrowing of the arteries is typically due to the buildup of plaque, or, in other words, an increase in the atherosclerotic burden. The buildup of plaque may also create a risk of stroke, heart attacks, and embolisms caused by fragments of plaque detaching from the artery wall and occluding smaller blood vessels. The risk of plaque detachment is particularly great when it is first deposited on the artery wall inasmuch as it is soft and easily fragmented at that stage.
Measurement of the atherosclerotic burden of the coronary artery itself is difficult and invasive. Moreover, assessment of risk often involves measuring both the atherosclerotic burden and its rate of progression. This assessment therefore involves multiple invasive procedures over time. Treatment of CAD also requires additional invasive procedures to measure a treatment's effectiveness.
The carotid artery, located in the neck close to the skin, has been shown to mirror the atherosclerotic burden of the coronary artery. Moreover, studies have shown that a reduction of the atherosclerotic burden of the coronary artery will parallel a similar reduction in the carotid artery.
One noninvasive method for measuring the atherosclerotic burden is the analysis of ultrasound images of the carotid artery. High resolution, B-mode ultrasonography is one adequate method of generating such images. Ultrasound images typically provide a digital image of the various layers comprising the carotid artery wall, which may then be measured to determine or estimate the extent of atherosclerosis. Other imaging systems may likewise provide digital images of the carotid artery, such as magnetic resonance imaging (MRI) and radio frequency imaging.
The wall of the carotid artery comprises the intima, which is closest to the blood flow and which thickens, or appears to thicken, with the deposit of fatty material and plaque; the media, which lies adjacent the intima and which thickens as a result of hypertension; and the adventitia, which provides structural support for the artery wall. The channel in which blood flows is the lumen. The combined thickness of the intima and media layers, or intima-media thickness (IMT), is reflective of the condition of the artery and can accurately identify or reflect early stages of atherosclerotic disease.
An ultrasound image typically comprises an array of pixels, each with a specific value corresponding to its intensity. The intensity (brightness) of a pixel corresponds to the density of the tissue it represents, with brighter pixels representing denser tissue. Different types of tissue, each with a different density, are therefore distinguishable in an ultrasonic image. The lumen, intima, media and adventitia may be identified in an ultrasound image due to their differing densities.
An ultrasound image is typically formed by emitting sound waves toward the tissue to be measured and measuring the intensity and phase of sound waves reflected from the tissue. This method of forming images is subject to limitations and errors. For example, images may be subject to noise from imperfect sensors. Another source of error is the attenuation of sound waves that reflect off tissue located deep within the body or beneath denser tissue. Random reflections from various objects or tissue boundaries, particularly due to the non-planar ultrasonic wave, may add noise also.
The limitations of ultrasonography complicate the interpretation of ultrasound images. Other systems designed to calculate IMT thickness reject accurate portions of the image when compensating for these limitations. Some IMT measurement systems will divide an image into columns and examine each column, looking for maxima, minima, or constant portions of the image in order to locate the layers of tissue comprising the artery wall. Such systems may reject an entire column of image data in which selected portions of the wall are not readily identifiable. This method fails to take advantage of other portions of the artery wall that are recognizable in the column. Furthermore, examining columns of pixels singly fails to take advantage of accurate information in neighboring columns from which one may extrapolate, interpolate, or otherwise guide searches for information within a column of pixels.
Another limitation of prior methods is that they fail to adequately limit the range of pixels searched in a column of pixels. Noise and poor image quality can cause any search for maxima, minima, or intensity gradients to yield results that are clearly erroneous. Limiting the field of search is a form of filtering that eliminates results that cannot possibly be accurate. Prior methods either do not limit the field searched for critical points or apply fixed constraints that are not customized, or even perhaps relevant, to the context of the image being analyzed.
What is needed is a method for measuring the IMT that compensates for limitations in ultrasonic imaging methods. It would be an advancement in the art to provide an IMT measurement method that compensates for noise and poor image quality while taking advantage of accurate information within each column of pixels. It would be a further advancement to provide a method for measuring the IMT that limited the field of search for critical points to regions where the actual tissue or tissue boundaries can possibly be located.